Improve visibility of font size chooser (#6988)

* Improve visibility of font size chooser

* Move slider dot sizes into variables

* Use a standard font size instead of percentage in Slider

* Use shorthand for padding in FontScalingPanel

* Change Slider text pos to px to be consistent when font changes

* Cleaner dot size for Slider selection

test/components/views/settings/__snapshots__/FontScalingPanel-test.tsx.snap

@@ -83,10 +83,16 @@ exports[`FontScalingPanel renders the font scaling UI 1`] = `
                     className="mx_Slider_selectionDot"
                     style={
                       Object {
-                        "left": "calc(-0.55em + 50%)",
+                        "left": "calc(-1.195em + 50%)",
                       }
                     }
-                  />
+                  >
+                    <div
+                      className="mx_Slider_selectionText"
+                    >
+                      15
+                    </div>
+                  </div>
                   <hr
                     style={
                       Object {